The present invention relates to a method for preparing an oxidation catalyst for the production of unsaturated carboxylic acids by vapor-phase oxidation of unsaturated aldehydes.
More particularly, the invention relates to a method for preparing an oxidation catalyst for the production of unsaturated carboxylic acids by vapor-phase oxidation of unsaturated aldehydes, said catalyst containing at least phosphorus, molybdenum and an alkali metal(s) as components, said method comprising dissolving or/and suspending raw materials for the catalyst in water, wherein the contents of ammonium ion and nitrate ion are controlled to be from 7 to 15 moles and from 0.1 to 4.0 moles, respectively, per 12 gram atoms of molybdenum immediately after mixing of all the raw materials that will constitute the catalyst.
A number of catalysts composed mainly of phosphorus and molybdenum or of their alkali metal salts have been proposed for the production of unsaturated carboxylic acids from unsaturated aldehydes by vapor-phase oxidation. For example, catalysts exhibiting excellent efficiency have been proposed by the present inventors in Japanese Patent Publication Nos. 29289/83 and 39138/83. In the preparation of catalysts containing the above-mentioned components, the selection of starting materials is generally important. There are commonly used ammonium paramolybdate as a molybdenum source and orthophosphoric acid as a phosphorus source. Although a method in which molybdenum trioxide is used in place of ammonium paramolybdate is described in an example of Japanese patent application Laid-Open No. 171444/82, this method has industrial disadvantages in that it requires a very long time for the preparation of molybdovanadophosphoric acid and the development of catalytic activity is unstable.
On the other hand, the catalyst often loses its activity during use. While various causes of this deactivation are conceivable, this deactivation is categorized as a type suddenly caused by an abnormal reaction in the course of the normal reaction and a type of gradual deterioration in the catalyst structure during long-term continuous operation. Sometimes the catalyst deterioration occurs before use by heat treatment at excessively high temperatures. It has been proposed that components of deactivated catalyst be recovered by chemical separation and purified to be reused, but this method is industrially uneconomical. There is another attempt to reclaim the deactivated catalyst by adding ammonia or the like to it as such, but the yield of reclamation is still unsatisfactory.